This invention relates to an injection molding machine adapted to molding of a resin material and, in particular, to a control system capable of improving the facility of operation executed by an operator.
Generally, injection molding of a resin product is carried out through a molding cycle comprising a resin melting process, a filling process, a holding process, and a cooling process. In order to obtain a molded product having an excellent quality, it is a common practice to control a temperature of a mold comprising a fixed mold and a movable mold, a temperature of a resin material within the mold, an injection pressure, and the like. In addition to such control, it is also important to control a clamping pressure applied to the mold and a mold opening which is a distance between opposite surfaces of the fixed mold and the movable mold. These opposite surfaces are called parting surfaces. Furthermore, it is important to determine a timing of switching from the filling process to the holding process, namely, V-P (velocity-pressure) switching. Thus, control of various factors is required to obtain a molded product having an excellent quality.
By way of example, description will be given as regards control of the clamping pressure. The clamping pressure is defined by a clamping force F which is calculated in accordance with the following equation: EQU F=A.cndot.P/1000,
where A represents a pressure receiving area (cm.sup.2) of the molded product while P represents an average internal pressure (kg/cm.sup.2) of the mold. In a conventional control system, the clamping pressure is held constant at a preselected clamping pressure throughout an injection process, including the filling process and the holding process, and the cooling process. In such a control system, however, various defects such as short shot, a weld mark or line, and burning are often caused to occur when the preselected clamping pressure is relatively high. Under the clamping pressure which is relatively high, air initially present in the mold when the mold is being filled with the melted resin and gas produced from the melted resin are not discharged from the mold at the end of the injection process. Such air and gas are compressed and remain in the mold. Under the circumstances, an operator decreases the preselected clamping pressure in order to expedite discharge of the air and the gas present within the mold. However, when the preselected clamping pressure is excessively decreased, there arises another problem of occurrence of flash.
Thus, the operator is required to do a lot of delicate work in determination and entry of various preselected values necessary to obtain the molded product having an excellent quality. Therefore, the operator must have long experience and great skill.